1. Field of the Invention
The present invention relates to a fuel cell stack comprising a plurality of fuel cell units each composed of an electrolyte interposed between an anode electrode and a cathode electrode, the plurality of fuel cell units being stacked with separators intervening therebetween.
2. Description of the Related Art
For example, the solid polymer electrolyte type fuel cell comprises a fuel cell unit including an anode electrode and a cathode electrode disposed opposingly on both sides of an electrolyte composed of a polymer ion exchange membrane (cation exchange membrane), the fuel cell unit being interposed between separators. Usually, the solid polymer type fuel cell is used as a fuel cell stack comprising a predetermined number of the fuel cell units and a predetermined number of the separators which are stacked with each other.
In such a fuel cell, a fuel gas such as a gas principally containing hydrogen (hereinafter referred to as "hydrogen-containing gas"), which is supplied to the anode electrode, is converted into hydrogen ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte which is appropriately humidified. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas principally containing oxygen (hereinafter referred to as "oxygen-containing gas") or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen gas are reacted with each other on the cathode electrode, and thus water is produced.
In order to supply the fuel gas and the oxygen-containing gas to the anode electrode and the cathode electrode respectively, a porous layer having conductivity, for example, porous carbon paper is usually disposed on the catalyst electrode layer (electrode surface), and the porous layer is supported by the separator. Further, one or a plurality of gas flow passages designed to have a uniform widthwise dimension are provided on the mutually opposing surfaces of each of the separators.
In such an arrangement, the condensed water or the water produced by the reaction exists in a state of liquid (water) in the gas flow passage in some cases. If the water is accumulated in the porous layer, the diffusion performance is lowered concerning the diffusion of the fuel gas and the oxygen-containing gas to the catalyst electrode layer. It is feared that the cell performance is conspicuously deteriorated.
In view of this fact, for example, a fuel cell is known, as disclosed in Japanese Laid-Open Patent Publication No. 6-267564, which comprises a fuel distributor plate having a fuel flow passage for supplying the fuel to the anode electrode, and an oxygen-containing gas distributor plate having an oxygen-containing gas flow passage for supplying the oxygen-containing gas to the cathode electrode, wherein at least one of the depth and the width of the oxygen-containing gas flow passage of the oxygen-containing gas distributor plate is gradually decreased from the upstream flow passage zone to the downstream flow passage zone of the oxygen-containing gas.
However, in order to sufficiently supply the fuel gas and the oxygen-containing gas to the electrode surface, the gas flow passage is provided in a meandering manner or in a circumscribing manner in the surface direction of the separator. For this reason, the gas flow passage is considerably lengthy in the surface of the separator. In the case of the conventional technique described above, the depth is large in the upstream flow passage zone of the oxygen-containing gas flow passage, and the separator itself is considerably thick-walled. Therefore, a problem is pointed out such that it is not easy to realize a small size of the entire fuel cell. Further, the conventional technique described above involves such a problem that the processing operation for gradually decreasing the depth from the upstream to the downstream of the gas flow passage is extremely complicated.